The problem of the automatic control of the operations being carried out by a plurality of videorecorders in a professional videorecording and playing system arises when operations such as video-signal editing, i.e., a processing allowing erasing replacement or superposition of determined sequences of said signal coming from one or more recorders, are to be effected.
To keep professional characteristics video-signal editing is to be made with at least a one frame accuracy. Besides each processing of the video signal must not alter the frame synchronism and must not affect the image e.g. by dropouts.
Video-recorders capable of these functions must have highly-professional characteristics, such as:
one frame accuracy of the positioning of the recording magnetic tape;
variable speed of the magnetic tape;
internal generation of a progressive digital code, to identify the images with a time progression and a one frame resolution, coded by hours, minutes, seconds, frames (1 sec=25 frames).
Particularly when the number of videorecorders involved is rather high and when the number of sequence editings is high, the automatic control proves a valid means to speed up and optimize the time taken to execute the operations as well as to minimize the error probability.
Furthermore in the big television production centers, where as a rule at least one computer already exists, it proves particularly useful and advantageous to it also for the automatic control of the videorecorder system. Thus, the computer, besides carrying out its usual tasks, can manage the automatic control as well as all the video-signal switchings among the transmitting and/or receiving units such as television cameras, studios, monitors.
The main problem arises from the necessity to realize highly autonomous interface circuits requiring the least information interchange possible with the computer, and consequently the shortest time occupation possible of the computer CPU (central processing unit).
A further problem is to make an automatic control system allowing the contemporaneous execution of a plurality of operations among different video-recorders.
Still another problem arises from the necessity to detect and signal errors which may be made while storing the data into the computer.
Finally there is the problem arising from the necessity to detect and correct misinterpretations by the videorecorders of data arriving from the computer.
At present an automatic control unit exists, the HPE1 by Ampex. This unit comprises a microcomputer allowing the control of 4 videorecorders; an internal memory is provided where data of 100 digital codes, identifying the entrance and exit points of sequences to be edited, can be stored. This unit performs one operation at a time, stopping after each operation and waiting for the command for the next operation; in case of an editing, the operator must set from outside the address (consisting of two decimal digits) to recall the code previously stored in the internal memory.
This operation can be effected by an external computer by a suitable interface placed between the control unit and said computer.
Yet a possible connection to an external computer is not convenient as this control unit is designed to operate autonomously, as it already comprises an intelligent microcomputer-based unit. Then the management of a dialogue between said unit and an external computer is excessive and complicated and requires too long CPU time.
E.g., it is too complicated to modify and set again the data of the sequences to be processed, once these data have been stored into said control unit.
Another disadvantage is that only one operation at a time can be executed and not two or more operations; besides said unit stops after each operation. Then it demands too many manual commands, thus increasing too much the overall operation time.